Fix the display of the floating review banners while scrolling.

The floating review draft banners had some issues when scrolling off the
edge of a review. Due to some imprecise calculations, the page would
appear to stutter for a brief moment while scrolling, as the banner
jumped back and forth between being docked to the page and floating over
the page.

To fix the stutter, we needed to set the placeholder to have the exact
height and margins of the original banner, pre-float. This kept the
container the same size. We also needed to fix some of the calculations
to not go in/out of float mode too soon.

With this fixed, the page no longer jumped, but the banner would just
disappear as we hit the edge of the review. Now, the banner will appear
to dock at the bottom of the review, and just scroll out of view as the
user scrolls further down.

The end result is a buttery-smooth scrolling experience.

Reviewed at https://reviews.reviewboard.org/r/6293/